To answer OP's question: yes, he is liable for the damage to the vehicle and yes, if Avis finds it, Avis will hit him up for the cost of the repair. Those costs can be extraordinarily high.

OP can risk it and hope that Avis misses it or he can repair or have it repaired. If indeed it's a tiny scratch, that's a minor issue.

The key here is for OP to honestly assess the damage. Saying it's a scratch when it's a dent, for instance, doesn't help him assess what to do.